This invention relates to rotary mechanical end face seals and more specifically to seals which include some form of pumping mechanism on one of the seal faces.
End face seals depend upon the constant flatness and parallelity of their cooperating radially disposed faces for the effectiveness of the seal produced by these faces. Initially, the faces are lapped flat and smooth so that when placed together, there is no space through which the fluid to be sealed can escape. In use, however, these faces are subjected to uneven wear because of (1) differing linear speeds of the faces produced by the differing radii of the faces; (2) lack of homogeneity of the material of the faces; (3) distortion of the faces by the pressure of the fluid on the rings on which the faces are formed; and (4) poor lubrication.
The first factor cannot be eliminated since it is inherent in the operation of two flat surfaces rubbing together with a rotating movement.
The second factor can be controlled to some extent, but since the material of the rings is determined by the nature of the fluid to be sealed, the choice of material is limited. In high pressure, large diameter water applications, one of the sealing rings is made of carbon. This material is difficult to mold in such manner that it is perfectly homogeneous in the large diameters and masses required to withstand the pressures encountered.
The third factor has lead to various balancing arrangements and to increases in the amount of material in the ring, the latter to provide greater stiffness to resist the pressure of the fluid. However, balancing is effective in an axial direction and has little or no effect upon radial pressure differences which are largely responsible for pressure distortion in the ring. Increasing the mass of the ring aggravates the difficulty in assuring homogeneity in the material of the ring.
The fourth factor is more difficult to detect than to remedy.
Over the years, seal designers have evolved certain design principles which serve to assist them in solving seal design problems. These are (a) increased seal balancing to reduce axial pressure on the seal faces and, hence, wear of said faces, (b) increased face width to reduce unit pressure on the faces, (c) zero turning moment of the ring and seat about the centroid of their axial sections at operating pressures to assure parallelity of the seal faces and, hence, even wear thereacross, and (d) the use of relieved areas (hydro-pads) on at least one seal face to draw lubricant between the faces when they move relative to one another. Commonly assigned, U.S. Pat. No. 4,407,512 to Trytek discloses such hyrdo-pads formed on primary ring seal face to draw lubricant between the faces of the mating ring and the primary ring to reduce friction between the faces.
The present invention is an improvement over the seal arrangement disclosed in U.S. Pat. No. 4,407,512, as well as, other non-contacting seal arrangements. It is expected to constitute an improvement and complement virtually every patterned face seal arrangement where modulation of the pumping effectiveness of the pattern is desired.
Use of a pattern on one face of a mechanical end face seal to create a pumping mechanism to arrangement movement of fluid between the faces is common. Such face patterns create hydrodynamic lift to create a fluid film between the faces. Seal arrangements are employed in pumps, compressors, mixers and other devices to retain a fluid such as a gas or liquid. Often the pumping mechanism is arranged to pump a buffer gas between the faces against the flow of the process fluid. U.S. Pat. Nos. 4,212,475; 4,407,512; 4,836,561; 5,092,617; 5,556,111; 5,938,206; 5,947,481; 6,446,976; 6,454,268; and 6,626,436 are but a few numerous seal arrangements that illustrate a variety of mechanical end face seal applications where a pumping mechanism is employed.
It has been found that for certain applications it is desirable to moderate the pumping effectiveness of the pumping mechanism. For example, in certain applications, it has been found that though a pumping mechanism on the face is important to seal operation and durability, its use can result in undesirable or excessive leakage.
The present invention provides a structure that tempers the effectiveness of the pumping mechanism and also provides a generally annular path about the seal face that permits uniform distribution of the fluid pumped by the pumping mechanism. In accordance with the present invention, a recessed surface is provided throughout the area which forms the pumping mechanism. That is, to say, in the known seal arrangements with a pumping mechanism such a hydro-pad seal, a wavy face seal or a spiral groove seal, the face includes a dam area and a face pattern area. The dam area is a axially outermost annular surface, which is in facing relation to the opposing seal ring. The face pattern area, which may be radially outward or radially inward from the dam area includes a pattern formed axially into the seal ring by some measurable amount from the plane of the face of the dam area. Depending on the pattern employed, the pattern area in prior known seal arrangements also includes areas which are coplanar with the plane of the dam area. Such is the case with a seal face with a spiral groove pumping mechanism or a hydro-pad arrangement.
The present invention embodies removal of some of the surface within the pattern area that would otherwise reside in the same plane with the dam area. The entire face pattern area is provided with a recessed surface that is displaced axially recessed relative to the plane of the annular dam surface in a direction away from the seal interface with the opposing ring seal face. This configuration provides a permanent gap between the pattern area and the opposing face formed on the other seal ring. The pattern then defines recessed areas that are recessed relative to the plane of the annular recessed area.
By providing the recessed area over the face pattern, the pumping effectiveness of the pumping mechanism is diminished somewhat. That result derives from the fact that the relatively rotating faces are spaced further apart than would be the case in a seal arrangement that did not embody the present invention. The space between the recessed surface and the face of the cooperating ring also provide a generally annular path for fluid distribution about the entire annular seal ring interface. This relationship improves lubrication and seal ring cooling and reduces seal face thermal distortion.
The present invention is considered suitable for use in virtually all patterned face seals including, without limitation, seals disclosed in the patents identified herein. It can be applied to a seal face with hydro-pads formed in the pumping mechanism area, or spiral grooves, “T” grooves, or other groove patterns or a wavy-tilt face pattern seal. It can be employed in a seal application that is I.D. (inside diameter) pressurized or O.D. (outside diameter) pressurized. It can be employed in seals that permit process fluid to flow across the seal ring interface or seal applications that pump a buffer fluid between the faces. The buffer could be a gas to seal against a gas or liquid process fluid or a liquid buffer to seal against a liquid process fluid. The advantages of the invention can be adapted to each of these known prior seal arrangements